This is what i did:
trace your hand with pencil on a piece of poster board... go down halfway up your forearm... Now go a little bigger (i went around 1/2 in.)... then you have your pattern...
as far as shirts go(if your puppet is wearing one)... try and find a size that fits your arm in the same color of the shirt you want your puppet to wear... make your long sleeve short sleeve... That may help some:

An alternate design is to have the sleeve (where the puppeteer's arm is inserted in to the "live arm" of the puppet) at the wrist, rather than the elbow. Where it goes really depends on how the puppet will be used and what is more comfortable for the puppeteer.

I did a performance once where we used one of our regular puppets as a human arm puppet by dressing he (the puppet) and I in white Oxford shirts. I stayed infront of the puppeteer manipulating the puppet and performed all of the hand actions without being attached at all. (Keep in mind we were working behind a 6' tall curtain) It looked good in practice on the monitor and when we reviewed performance footage.

This is a quick and easy option if you find yourself in need of a human character.

Sounds similar to the method I use with my makeshift live-hands: I pull a long-sleeved shirt over the puppet's head so he's wearing it, then I stick my arm up through the shirt's bottom opening and the left sleeve, then add an appropriately-colored glove if needed. A somewhat limited range of movement, but it works well for temporarily converting an already-existing puppet into a live-hand.